Technical Field
The present disclosure relates to the field of wireless communication, and in particular, to device-to-device (D2D) wireless communication methods and user equipments (UEs) therefor.
Description of the Related Art
Device-to-device (D2D) wireless communication is a new topic in 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) Release 12. D2D communication could happen with wireless network coverage (e.g., for commercial case) or without network coverage (e.g., for public safety). FIG. 1 illustrates exemplary D2D communications with and without wireless network coverage. On the left side of FIG. 1, UE 101 and UE 102 are within the wireless network coverage of eNB (eNode B) 103, but they are communicating with each other directly (i.e., not through eNB 103). On the right side of FIG. 1, UE 104 and UE 105 are not within any wireless network coverage, and they are communicating with each other directly.
When UEs with D2D communication capability (D2D UEs) are with wireless network coverage, i.e., in LTE Wide Area Network (WAN), D2D UEs may operate with LTE WAN and D2D simultaneously, which means that in some radio resources/subframes, D2D UEs transmit/receive LTE WAN signals, but in other radio resources/subframes, D2D UEs transmit/receive D2D signals. Currently, from resource allocation point of view, D2D UEs can operate in one of the following two modes (refer to LTE Rel. 12) for D2D communication:                Mode 1: eNodeB or LTE rel-10 relay node schedules the exact resources used by a UE to transmit direct data and direct control information;        Mode 2: a UE on its own selects resources from resource pools to transmit direct data and direct control information.        
In mode 1, the resources of D2D transmission are allocated or fully controlled by an eNB, but in mode 2, the UE itself selects the resources for D2D transmission.